1. Field of the Invention
The present invention relates to an operational amplifier method and an operational amplifier. It specifically relates to an operational amplifier circuit with a low dissipation current and a large drive current, which is powered by a low voltage power supply.
2. Description of the Related Art
Recently, to embed in a system LSI (Large Scale Integrated circuit) for a portable apparatus such as an electronic notebook or a digital watch, powered by a primary battery, the CMOS (Complementary Metal Oxide Semiconductor) operational amplifier circuit (which has been made up using a CMOS manufacturing process) is frequently utilized.
These portable devices driven by the battery have to function with dissipation currents ranging from several microamperes to several milliamperes with the help of a power supply voltage of 1.5 volts or less. Accordingly, an operational amplifier circuit, where functions are powered by a low power supply voltage with a low dissipation current, is required.
In addition, an operational amplifier circuit with a high output capability as well as a capability of operating with a low dissipation current, powered by a low voltage supplied by a power supply, is also in demand for a portable apparatus. It is desired, for example, to further decrease a dissipation current spent in a portable apparatus during its steady-state operation, and drive a loudspeaker via the operational amplifier circuit by the output of a D/A (Digital to Analog) converter, which along with an A/D (Analog to digital) converter has been embedded on the same chip as the operational amplifier circuit.
To respond to the request, a folded cascode-type operational amplifier circuit 700 (hereafter, referred to as just "operational amplifier circuit") as shown in FIG. 1 has been proposed.
The operational amplifier circuit 700 is comprised of: a folded cascode-type operational amplifier circuit 71 including n-channel transistors 74 and 75 which make up a pair of differential transistors and a current mirror circuit 76. Also included is push-pull output circuit 72 including a p-channel output transistor 77, a n-channel output transistor 78, a condenser for phase compensation Cc, and an output terminal 79. A bias circuit 73 supplies the folded cascade circuit 71 and the push-pull output circuit 72 with a bias voltage.
Upon application of an input signal to gates G74 and G75 of the n-channel transistors 74 and 75 which make up a pair of differential transistors, a signal amplified by the folded cascode circuit 71 is output to a common gate terminal A of the current mirror circuit 76 and an output terminal B of the folded cascode circuit 1, amplified by the p-channel output transistor 77 and the n-channel output transistor 78, and output to the output terminal 79.
The conventional operational amplifier 700 as described above has a feature where: the range of input voltages is wide; the range of output voltages is extended to the earth voltage; and it is possible to function with the help of a low voltage of 1 volt or less supplied by a power supply.
However, the operational amplifier circuit 700 is not configured in such a manner that the p-channel output transistor 77 is driven by the output of the differential amplifier circuit. In addition, it also does not comprise a circuit configuration used in setting an idling current for the n-channel output transistor 78.
Thus, there is a shortcoming with the operational amplifier circuit 700 where the push-pull output circuit 72 operates as a class A amplifier so that the maximum output current of the operational amplifier circuit 700 is limited to an amount which is equal to or less than the idling current flowing through the p-channel output transistor 77.
Furthermore, when the push-pull output circuit 72 operates and outputs a high amplitude of signal, the shape of the high amplitude of signal is asymmetric with respect to the middle point of voltage. This causes an increase in an odd-order harmonic distortion.